if you only have a slight leak the carpet wont be wet, but the bottom of the sound proofing underneath will be
when my car was warm the windows misted up very quickly, couldnt find a leak anywhere, lifted carpets up and found it was damp, heater matrix had sprung a tiny leak causing the misting up
so check behind your dash, dont know where it is on the focus im afraid, but its worth lifting carpets at bulkhead and searching, as when heater matrix doe go 98% they leak into the car